Glass frit bond line

ABSTRACT

A lead-containing glass material of the type suitable for use in a wafer bonding process, wherein the moisture resistance of the glass material is increased by the presence of a lead phosphate coating on an outer exposed surface of the material, thereby acting as a barrier to reaction of moisture with the lead of the glass material. A source of reactive phosphate ions is applied to the glass material so as to spontaneously form the desired lead phosphate coating.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention generally relates to glass frit compositions. Moreparticularly, this invention relates to lead-containing glass fritmaterials of the type suitable for use in wafer bonding processes,wherein the moisture resistance of the glass frit material after firingis increased by a lead phosphate coating formed on an exposed outersurface of the glass frit material.

2. Description of the Related Art

Within the semiconductor industry, there are numerous applications thatrequire bonding a semiconductor wafer to a second wafer or glasssubstrate. As an example, a microelectromechanical system (MEMS) deviceformed in or on a semiconductor wafer (referred to herein as a devicewafer) is often capped by a semiconductor or glass wafer (referred toherein as a capping wafer), forming a package that defines a cavitywithin which the MEMS device, such as a suspended diaphragm or mass, isenclosed and protected. Examples of MEMS devices protected in thismanner include accelerometers, rate sensors, actuators, pressuresensors, etc. By the very nature of their operation, MEMS devices mustbe free to move to some degree, necessitating that the seal between thewafers is sufficient to exclude foreign matter from the cavity. CertainMEMS devices, such as absolute pressure sensors, further require thatthe cavity be evacuated and hermetically sealed. The performance ofmotion sensors and accelerometers with resonating micromachinedcomponents also generally benefit if the cavity is evacuated so thattheir micromachined components operate in a vacuum. A hermetical sealalso ensures that moisture is excluded from the cavity, which mightotherwise form ice crystals at low temperatures that could impede motionof the MEMS device.

In view of the above, the integrity of the bond that secures the cappingwafer to the device wafer is essential to the performance and life ofthe enclosed MEMS device. Various bonding techniques have been used forthe purpose of maximizing the strength and reliability of the waferbond. Such techniques include the use of various intermediate bondingmaterials, including glass frit, as well as silicon direct and anodicbonding techniques that do not require an intermediate bonding material.As would be expected, each of these bonding techniques can beincompatible or less than ideal for certain applications. Silicon directand anodic bonding methods require very smooth bonding surfaces, andtherefore cannot produce a vacuum seal when trench isolation orunplanarized metal crossunders are employed on the device wafer, such asto electrically interconnect a MEMS device to bond pads outside thevacuum-sealed cavity of the package. In contrast, glass frit and otherintermediate bonding materials are able to form suitable bonds withdeposited layers, runners and other surface discontinuities often foundon device wafers.

Glass frit bonding materials used for wafer bonding are often depositedby a screen printing technique, in which case the material is depositedas a paste that contains a particulate glass frit material, athixotropic binder, and a solvent for the binder. The proportions ofglass frit, binder and solvent are adjusted to allow screen printing ofa controlled volume of the paste on a designated bonding surface of oneof the wafers, typically on the capping wafer. After firing, the cappingand device wafers are aligned and then mated so that the glass fritparticles (bonded together as a result of the firing operation) contacta complementary bonding surface of the second (e.g., device) wafer. Thewafers are then incrementally heated to completely remove the solventand binder and finally melt the glass frit, so that on cooling the glassfrit material resolidifies to form a substantially homogeneous glassbond line between the wafers.

The composition and size of a glass frit material used in a waferbonding process are typically chosen on the basis of process andcompositional considerations, including screening properties, processtemperatures, etc. Suitable glass frit materials are usually a mixtureof various oxides, such as litharge (PbO; also known as lead oxide,yellow and lead monoxide), boric acid (H₃BO₃) which serves as a sourcefor boron oxide (B₂O₃), silicon dioxide (SiO₂; silica), aluminum oxide(Al₂O₃, alumina), titanium oxide (TiO₂, titania), cupric oxide (CuO),manganese dioxide (MnO₂) or manganese carbonate (MnCO₃) as a source formanganous oxide (MnO), calcia (CaO), lithium oxide (Li₂O), ceria (CeO₂),cobaltous carbonate (CoCO₃), and others. Glass frits that contain leadare widely used in the semiconductor industry to enable silicon waferbonding at temperatures sufficiently low to reduce the risk of thermaldamage. However, the percentage of lead required in a glass fritmaterial to achieve high quality wafer-level sealing at low firingtemperatures makes the resulting glass susceptible to attack bymoisture. When a lead-containing glass is exposed to moisture, lead canbe released from the glass and, in concert with moisture, migrate toexposed aluminum metallization on the wafer, resulting in galvaniccorrosion of the metallization. This corrosion is a source of concernfor both the wafer-level yield and the system-level reliability of asemiconductor device.

Prolonged exposure to moisture can also create holes in a glass bondline that are potential paths for the ingress of air, moisture andcontaminants into a cavity sealed by the bond line. Glass frit used inwafer bonding processes is often exposed to attack by moisture as aresult of the use of water to cool the wafers and remove debris duringthe dicing operation used to singulate dies from the wafers. Duringsingulation, the glass bond line is generally covered by water for theduration of the dicing operation, making the glass-water interactiondifficult to avoid. Following dicing, if the device is placed in apackage that is not hermetically sealed, the glass bond line may besubjected to prolonged exposure to moisture during the operating life ofthe device.

In view of the above, it would be desirable if lead-containing glassfrit materials of the type used in wafer bonding processes could berendered more resistance to attack by moisture.

SUMMARY OF THE INVENTION

The present invention is directed to lead-containing glass fritmaterials of the type suitable for use in wafer bonding processes,wherein the moisture resistance of the glass frit material is increasedby the presence of one or more lead phosphate compounds. The leadphosphate compounds are preferably in the form of an outer coating thatprotects the surface of the glass frit material, thereby acting as abarrier to reaction of moisture with the lead contained by the material.

A suitable method for making and using the glass frit material of thisinvention comprises depositing the lead-containing glass frit materialon a surface. The glass frit material, comprising glass frit particles,is then fired to melt and fuse (bond) the particles, after which asource of reactive phosphate ions is applied to the bonded glass frit soas to spontaneously form the desired lead phosphate coating on theexposed surface of the glass frit.

Glass frit materials protected with a lead phosphate coating inaccordance with this invention are particularly well suited for bondingdevice and capping chips together, such as where the glass frit materialis used to hermetically seal a cavity defined by and between the devicechip and the capping chip. In such an application, the lead phosphatecoating prevents the lead content of the glass frit material fromreacting with moisture, such that lead is not released to react withaluminum metallization on the chips and create holes that would degradethe hermetical seal formed by the glass frit material.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a MEMS device package that includesa capping wafer glass frit bonded to a device wafer in accordance withthis invention.

DETAILED DESCRIPTION

FIG. 1 represents a portion of a MEMS device package 10 formed by glassfrit bonding a device wafer 12 to a capping wafer 14, such that amicromachined structure 16 (schematically represented in FIG. 1) isenclosed within a cavity 18 between the wafers 12 and 14. The devicewafer 12 is typically formed of a semiconductor material such assilicon, preferably monocrystallographic silicon, though it isforeseeable that other materials could be used. The capping wafer 14 maybe formed of glass, ceramic, or a semiconducting material. Themicromachined structure 16 can be of any desired type, such as a proofmass, resonating structure, diaphragm or cantilever that relies oncapacitive, piezoresistive and piezoelectric sensing elements to senseacceleration, motion, pressure, etc., all of which are known in the art.

As is conventional, the micromachined structure 16 is electricallyinterconnected to metal bond pads 20 on the device wafer 12 byconductive runners 22, often in the form of aluminum metallization.Through the bond pads 20, the micromachined structure 16 and itsassociated sensing elements can be electrically interconnected withappropriate signal conditioning circuitry (not shown), which may beformed on the device wafer 12, the capping wafer 14 or a separate chip.The runners 22 cross through a bonding surface 26 contacted by a glassbond line 30 that bonds a corresponding bonding surface 28 of thecapping wafer 14 to the bonding surface 26 of the device wafer 12.

According to common practice, the bond line 30 is formed by firing apaste composition containing a glass frit material and, typically, abinder and solvent. The binder and solvent are removed and the glassfrit material is melted by sufficiently heating the paste, with theresult that essentially only glass remains to form the bond line 30.Suitable binders and solvents for the paste are those used incommercially-available glass frit paste mixtures, and have vaporizationtemperatures well below the softening point of glass frit materialssuitable for the paste (e.g., about 355° C.). As known in the art, theproportions of the glass frit material, binder and solvent are chosen atleast in part to achieve the desired deposition resolution for thepaste, and consequently, the minimum width and thickness of the glassbond line 30. A suitable deposition method is screen printing inaccordance with known practices, by which the paste is deposited througha mask or screen to form a thick film on the bonding surface 26 or 28 ofthe device or capping wafer 12 or 14. The wafer 12 or 14 is heated toremove the solvent and binder from the paste, and then soften or meltthe surfaces of the glass frit particles to bond or tack the particlestogether. After cooling to resolidify the glass frit material, thecapping and device wafers 12 and 14 are aligned and mated so that theglass frit material is between and contacts both bonding surfaces 26 and28 of the wafers 12 and 14. The wafers 12 and 14 are then heated to atemperature sufficient to remelt the glass frit material, after whichthe wafers 12 and 14 are cooled to resolidify the glass frit materialand form the glass bond line 30 shown between the wafers 12 and 14 inFIG. 1.

The present invention is concerned with glass bonds formed from glassfrit compositions that contain lead, such as from lead oxide (PbO), alsoknown as litharge, and yellow and lead monoxide. Various other materialsmay be present in the frit composition, including but not limited toboric acid (H₃BO₃) which serves as a source for boron oxide (B₂O₃),silicon dioxide (SiO₂; silica), aluminum oxide (Al₂O₃, alumina),titanium oxide (TiO₂, titania), cupric oxide (CuO), manganese dioxide(MnO₂) or manganese carbonate (MnCO₃) as a source for manganous oxide(MnO), calcia (CaO), lithium oxide (Li₂O), ceria (CeO₂), cobaltouscarbonate (CoCO₃). However, it is the lead content in the glass bondline 30 formed by firing the glass frit that renders the bond line 30susceptible to attack by moisture. As a result of exposure to moisture,elemental lead can be released from the bond line 30 and migrate to thealuminum runners 22 on the device wafer 12 to cause galvanic corrosion.Lead migration from prolonged exposure to moisture, such as duringdicing when the device package 10 is singulated from a wafer stack, canresult in the creation of holes in the bond line 30 that are paths forthe ingress of air, moisture and contaminants into the cavity 18, whichis often required to be hermetically sealed by the bond line 30.

To prevent or at least inhibit the depletion of lead from the bond line30, a source of reactive phosphate ions is applied to the bond line 30(i.e., after firing of the glass frit paste), which results in theformation of a lead phosphate coating on the exposed outer surface ofthe bond line 30. An example of a suitable source of phosphate ions is asurfactant commercially available from Union Carbide under the nameTriton X-100, which contains a trace amount of phosphates, believed tobe on the order of parts per million or less. The Triton X-100surfactant has been successfully used in a diluted form by preparing asolution of about five parts by volume of the surfactant to about 95parts of deionized water, though solutions containing as little as about2.5 parts by volume of the surfactant, the balance water, are believedto be effective. While the Triton X-100 has been found to be a suitablesource of reactive phosphate ions, other sources containing a sufficientamount of reactive phosphates—again, even amounts measured in parts permillion (e.g., less than 0.001%) or less—could be used, as long as thesource is able to deliver phosphate ions to the glass surface so thatlead present in the glass is reacted to form a protective lead phosphatecoating.

Phosphate ions are believed to bond with lead through several possiblereactions, such as:

3Pb⁺²+2(PO₄)⁻³→Pb₃(PO₄)₂

5Pb⁺²+3(PO₄)⁻³+H₂O→Pb₅(PO₄)₂OH+H⁺

These reactions tie up lead on the surface of the bond line 30 byforming lead phosphates that are highly insoluble in water (log Kspvalues of 44.36 and 76.8, respectively). Through these reactions, anylead-containing glass frit material (e.g., bond line 30) will develop aprotective coating containing one or more lead phosphates. The leadphosphate coating acts as a moisture barrier to the bulk of the glassfrit material, thereby inhibiting attack and greatly decreasing theprobability of device failure due to moisture-induced degradation of theglass frit.

During an investigation leading to this invention, a solution wasprepared containing the Triton X-100 surfactant and deionized water in avolume ratio of 5/95. Two identical accelerometer packages were obtainedhaving an acceleration-responsive proof mass enclosed within a cavityhermetically sealed with a glass bond, similar to what is depicted inFIG. 1. The glass bond was formed by firing a glass frit materialcontaining lead oxide particles. Both packages also included runners andbond pads formed by aluminum metallization and contacted by the glassbond. One of the packages was treated in accordance with this inventionby immersing the package in the surfactant solution for about tenminutes, such that the exposed portions of the glass bond were contactedby the solution. Both packages were then immersed in deionized water ata temperature of about 95° C. for about 2.5 hours.

Examination of the untreated package evidenced that a large amount oflead had been released from the glass bond, and that the aluminummetallizations were darkened by what was determined by energy dispersivex-ray (EDX) analysis to be a layer of lead oxide. In contrast, the glassbond of the treated package did not suffer any discernable loss of lead,and the surfaces of the aluminum metallizations were free of lead oxidecontamination. Microphotographs of the glass bond lines of both packagesevidenced that the untreated glass bond had a roughened, crystallineappearance with holes attributed to the loss of lead. In contrast, thebond line of the treated package had a smooth texture with a feathery“skin” identified by EDX as phosphorus-rich and attributed to leadphosphates formed by the surfactant treatment. The lead phosphate skincompletely covered the exposed surface of the glass bond line. Fromthese results, it was concluded that the lead phosphate coating waspresent in an amount sufficient to prevent the lead oxide of the glassbond line from reacting with the hot water bath. As such, it wasconcluded that treating a lead-containing glass frit material with aphosphate-containing source is capable of preventing the loss of leadfrom the frit material, and therefore capable of avoiding degradation ofthe glass frit bond and contamination of surrounding die surfaces.

While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. For example, glass frit material treated inaccordance with this invention could be used in a variety of otherapplications, including displays, etc. Accordingly, the scope of theinvention is to be limited only by the following claims.

What is claimed is:
 1. A bond line bonding a device chip to a cappingchip, the bond line consisting essentially of a lead-containing glassmaterial, the bond line having an outer exposed surface defined at leastin part by a coating comprising lead phosphate.
 2. The bond lineaccording to claim 1, wherein the glass material consists of lead oxideand at least one material chosen from the group consisting of boronoxide, silicon dioxide, aluminum oxide, titanium oxide, cupric oxide,manganous oxide, calcia, lithium oxide, ceria, and cobaltous carbonate.3. The bond line according to claim 1, wherein the bond line is on asemiconductor wafer.
 4. The bond line according to claim 1, wherein thebond line hermetically seals a cavity defined by and between the devicechip and the capping chip.
 5. The bond line according to claim 1,wherein the coating completely defines the other exposed surface of thebond line and the lead phosphate is present in the coating in an amountsufficient to prevent lead within the bond line from reacting withmoisture.
 6. A bond line bonding a device chip to a capping chip, thebond line consisting essentially of a glass material consisting of alead oxide and at least one material chosen from the group consisting ofboron oxide, silicon dioxide, aluminum oxide, titanium oxide, cupricoxide, manganous oxide, calcia, lithium oxide, ceria, and cobaltouscarbonate, the bond line having an outer surface completely covered by alead phosphate coating.
 7. The bond line according to claim 6, whereinthe bond line hermetically seals a cavity defined by and between thedevice chip and the capping chip.
 8. The bond line according to claim 7,wherein the cavity contains a microelectromechanical device.
 9. The bondline according to claim 6, wherein the device chip has aluminummetallization contacting the bond line.